Tokkai Hei 9-228828 published by the Japanese Patent Office in 1997 discloses the reduction of hydrocarbons (hereafter HC) emissions immediately after engine start-up by the provision in the exhaust pipe of an HC absorbent catalytic converter, which is comprised of an HC absorbent material and a three-way catalyst. The HC absorbent material absorbs HC when the catalyst temperature is lower than a fixed temperature and emits HC at temperatures higher than the fixed temperature.
In such a way, HC emitted from the engine is temporarily absorbed by the HC absorbent material in the HC absorbent catalytic converter. When the temperature of exhaust gases in the HC absorbent catalytic converter reaches a fixed temperature, the air-fuel ratio is varied to a lean air-fuel ratio, that is to say, the air-fuel ratio is varied upwardly. This results in a super-oxygenated mixture in the exhaust gases and HC which is emitted from the HC absorbent material is oxidized by the three-way catalyst.